There is the standard classic revenge, but it entails law breaking, so anything I say or said is hypothetical only.
Break into their houses and tamper with their stuff, then replace the stuff so it looks undisturbed , and then leave.
This works well if you get their toothbrush, and give the toilet rim a bit of a scrub with it, itch your anus with it, comb the dog's arse hair with it, jizz on it, ...whatever. The key here is to take a picture of their toothbrush as you are brushing the cat's balls with it. Then, when the timing feels right, email or post them a print of the pic. Make sure it has identifying background items in so they know someone was in their house, interfering with their stuff, and that the toothbrush in the photo being used to floss a bum's armpit hair, is indeed theirs.
For the ladies, any kind of yeast/mushroom dust/dried fecal dust sprinkled onto sanitary pads is very nasty.
Poking pinholes in condoms is another goodie.
Plant dubious porn or sexual toys in places likely to be discovered by irate partners/parents.
Using a very small needle size, use a syringe to inject your urine into plastic bottled drinks/shampoo/dishwash/ or into cuts of meat/spreads/alcohol etc... make sure to capture your 'urine placements' on camera.
If you can manage to collect them, infest their house or underwear drawer with fleas/tics/roaches.
Dip ALL their eating utensils in the toilet, let them dry, replace.
A turd or dead bird in the water pipes somewhere if possible. Particularly easy if their house uses a hot water 'header' tank.
A light dried coating of dishwashing liquid on cups and plates usually leads to diarrhea. Or you could mix Turbo-Lax into whatever you can find.
Thin pins or sewing needles slotted into the carpet on an almost horizontal angle are hard to see and will hurt like hell.
A loop connecting the + and - pins on power cords gives a nice BANG when plugged in. Fuse wire is good.
Chew some gum, the softer the gum stays the better, then put it on the inside of their caps/hats. Hopefully they won't notice til they have to get it cut out.
There are certain dusts/powders of a fine consistency that are very irritating to human skin. These are perfect for adding to toilet rolls.
The best thing you could do, with access to the right information, is assume their identity.
Oh, and take ALL the remotes.
